This invention relates to the manufacture of thermoplastic films. More particularly, this invention relates to thermoplastic films having minute perforations on a regularly arranged pattern over substantially the entire surface of the film which have increased liquid flow rates through their perforations and are useful in making sanitary and apparel structures or assemblies, and particularly for making infant and/or adult diapers.
Many thermoplastic films of different strengths, characteristics, colors, and dimensions are produced for a multitude of products. Perforated films have a multiplicity of regularly and/or randomly spaced apertures which allow permeation of liquid and air or other fluids. Such films can be used as a component of disposable composition garments for sanitary apparel purposes, such as napkins, diapers, or for hospital pads, bed or sleeping bag linings, and the like. In such composition structures, an exterior layer of film having the desired properties is provided which would be adjacent to the skin in a composite garment, and the garment would also include a filler layer or layers of absorbent fibrous material.
Surface treating of film for rendering the surfaces of the film more receptive to adhesives, paints, inks, lacquers, or to other surfaces in laminating processes is widely practiced throughout the plastic film industry. Various types of processes are utilized. Among these are corona discharge treatment, chemical treatment, flame treating, casing, electronic treating, plasma etching, ion plating and irradiation.
Chemical treatments include antistatic agents, additives which alter the polarity of the surface, adhesion promoters and other surface modifiers. A wide variety of additives for plasticizing, coloring, impact modification, slip, anti-fogging and stabilization may be added to the film forming resin prior to the film being made. Such chemical compounds may be added batchwise in powder mixers or to the feeding mix to compounders such as Banbury mixers or compounders.
A particular class of perforated film is described in Thompson, U.S. Pat. No. 3,929,135, issued Dec. 30, 1975. Thompson teaches an absorptive structure with a top layer of perforated film characterized by having a series of regular spaced small apertures in the form of tapered capillaries of certain dimension ranges. In the finished article, these are directed inwardly to be in intimate contact with an absorbent fibrous material layer. The smooth side of the perforated film is thus in use in contact with the skin. Film as described by Thompson, in a garment structure as outlined, maintains a dry and comfortable condition, even after transmission of fluids to the absorbent layer by the combined effects of the absorption and the resistance to back flow as a result of the relative length and surface properties of the tapered capillaries.
One method for making film with tapered capillaries on one side thereof is shown in U.S. Pat. No. 3,054,148, issued Sept. 18, 1961, to Zimmerli. In this patent heated film is supported by a perforated screen and a vacuum applied to the underside of the perforated screen. Holes are pulled in the film in the direction of the vacuum beneath the screen thereby forming tapered capillaries in the film.
An alternative technique for forming tapered capillaries is disclosed in U.S. Pat. No. 3,394,211, issued July 23, 1968, to McDuff. In the McDuff process the perforation technique comprises applying a blast of heated air to create a pressure differential across a perforated forming surface, conditions being appropriate to cause a rupture at a desired point.
Furthermore, film may be perforated by a variety of other means including mechanical perforation by pins or the like, and by electrical means such as corona discharge. U.S. Pat. No. 3,471,597 discloses a method for perforating a film by corona discharge and U.S. Pat. No. 3,880,966 discloses a method of increasing the permeability of a film with corona discharge. However, some, if not all, of such films do not have tapered capillaries.
Mito et al U.S. Pat. No. 2,763,759 discloses an apparatus for electrically perforating polymer sheet material or film. The film is first deformed with needles and then subjected to electrical discharge treatment to perforate the film.
U.S. Pat. No. 3,703,569 issued to Wolkowicz describes a corona treatment of a sheet polyethylene containing an antistatic agent, whereby the static decay rate of the sheet is increased.
Kozak, U.S. Pat. No. 3,814,101 discloses a type of perforated film having valvular openings or slits and a diaper incorporating such film.
Recently issued U.S. Pat. No. 4,283,291 to Lowther describes an apparatus for providing a corona discharge.
Canadian Pat. Nos. 766,213 and 790,038 to Adams et al and U.S. Pat. No. 3,018,189 to Traver disclose method and apparatus for corona discharge surface treatment of polyethylene film for improving the adhesion of dried ink impressions subsequently imprinted on the film. U.S. Pat. No. 3,754,117 to Walter discloses another type of apparatus and method for corona discharge treatment for modifying the surface properties of thin layers or fibers for similar adhesion improvement of inks or paints or of subsequent bonding.
U.S. Pat. No. 4,170,304 to Huke describes a method of manufacturing a polyethylene-polyisobutylene wrapping film employing a corona discharge surface treatment for improving the adhesive characteristic of the film.
Copending application Ser. No. 216,219 filed Dec. 15, 1980 now U.S. Pat. No. 4,351,784 discloses a corona discharge surface treatment of perforated thermoplastic film having tapered capillaries for increasing the flow of fluids through the film.
Thompson, supra, suggests that cell geometry, i.e. the specific shape of the tapered capillaries is the principal variable yielding good fluid flow rate or percent run off. Deviations from such critical shape will diminish the flow rate. While ideal dimensions may provide maximum flow rate, they are difficult to achieve economically in commercial operations.
Wetting agents such as soaps, detergents and surface-active agents are widely used in polymerization reactions and in preparing emulsions of plastic. Such compounds cause a liquid to penetrate more easily into or to spread over the surface of another material. Surface active agents or surfactants are compounds that alter the surface tension of a liquid in which they are dissolved.
Surface tension is the contractile surface force of a liquid by which it tends to assume a spherical form and to present the least possible surface. It may also be exerted at the junction of two liquids. It is generally measured in dynes per centimeter.
In general, to manufacture perforated polyethylene film having tapered capillaries of the critical dimensions welded cylinder screens are used. Such screens are expensive and have a relatively short life span. Replacing the screens with less expensive and longer lasting non-welded screens results in a product having a somewhat different cross-sectional geometry than desired and a higher percent run off.
Film produced on welded screens has substantially the desired cell geometry and normally has a zero or near zero percent run off. Film produced on non-welded screens has a less desirable cell geometry and as much as 10 percent run off only after the film is heavily treated with corona discharge. This difference in percent run off is the difference between a highly marketable product and an undesirable one.
It has been discovered that by combining a chemical surface treatment wherein a surfactant is added or blended with the resin, with a corona discharge treatment a perforated polyethylene film having a zero or near zero percent run off can be produced on the less expensive non-welded screens.
It is therefore an important object of the present invention to provide a relatively economical means of producing a perforated film having zero percent run off.
It is an object of the present invention to increase the flow of liquids through a perforated film having tapered capillaries of less than ideal dimensions or cell geometry.
Other objects and advantages of the invention will become more readily apparent from a consideration of the description and drawings hereinafter.